It is generally known in agricultural sciences that sulphur fertilization increases crop yield and quality and further has a beneficial effect on nitrogen processing by plants. This nitrogen processing is in turn related to protein synthesis, cell replication, photosynthesis, and disease resistance.
However, more stringent air pollution regulations and the reduction of sulphur dioxide emissions have resulted in a greater incidence of sulphur deficiencies in agricultural soils, thereby increasing the demand for sulphur containing fertilizers.
Generally speaking, powdered sulphur has little utility as an agricultural fertilizer because of difficulty in handling, and the sulphur dust acts as an eye irritant. Sulphur dust also presents an explosive risk in handling facilities and segregates if blended with granular fertilizer products.
Accordingly, most agricultural fertilizers used in field applications are formulated into particles in order to alleviate these drawbacks. Particulate products can be easily handled without the generation of excessive dust and segregate less when blended with other particulate products.
Conventionally, sulphur has been applied in the form of elemental sulphur, ammonium sulfate, ammonium thiosulfate, ammonium bisulfate, sulfides or calcium sulfate (gypsum).
Due to the high analysis of elemental sulphur fertilizer products, they are more cost effective than sulfate products because of lower transportation, handling, and storage costs. High analysis fertilizer products also allow more flexibility when blending with other nutrient products. The main drawback of elemental sulphur products is that their oxidation to plant available sulphate is slow often taking many years from the time of application.
The effectiveness of elemental sulphur fertilizers depends on the rate of their oxidation to the plant-available sulfate form. In soil, oxidation to sulphate is a microbial process involving bacterial species such as Thiobacillus. The rate of this reaction is strongly influenced by soil and environmental factors as well as the physical and chemical properties of the fertilizer product. Sulphur oxidation is generally a surface reaction. Thus, the amount of sulphate produced per unit of time is a function of the total surface area of elemental sulphur present, not of its mass, hence the rate of oxidation is strongly influenced by sulphur particle size.
Two physical attributes of fertilizer products that have a substantial effect on oxidation rates are the extent of dispersion from the fertilizer granule after it has been introduced on or in the soil and the size of the dispersed sulphur particles, with smaller particles oxidizing more rapidly than larger particles.
In order to provide greater dispersion of the sulphur some elemental sulphur products mix the sulphur with bentonite clay by means of a hot process utilizing molten sulphur during the production stage. Upon application, the clay functions to imbibe water and break apart or disperse the particulate. In actuality the particulates do not readily disperse into finely divided particles and sulphur oxidation proceeds at a very slow rate. This occurs due to molten hydrophobic sulphur coating the clay particles during the production stage preventing contact with water.
Thus, it would be advantageous to have a granular elemental sulphur containing fertilizer which has a high nutrient content and quickly disperses into finely divided particles, say for example sub 30 micron particles, once contacted with water on or in the soil.
Various products and processes have heretofore been proposed. For example, U.S. Pat. No. 4,133,669 relates to a process for pelletizing a mixture of elemental sulphur and bentonite clay to produce a water degradable prill which includes adding dry clay dust to molten sulphur at an elevated temperature to provide a molten sulphur-clay mixture, forming droplets of the mixture, providing a liquid fertilizer coolant at a temperature lower than the freezing temperature of the sulphur-bentonite mixture, feeding the droplets into the coolant, passing the droplets through the coolant for a time sufficient to anneal the droplets into pellets, and removing the annealed pellets from the coolant.
Furthermore, U.S. Pat. No. 4,330,319 relates to a process for the production of a urea sulphur fertilizer by mixing urea and molten sulphur to obtain a molten mixture and solidifying the molten mixture to obtain a homogeneous, solid, particulate urea sulphur fertilizer wherein the sulphur has particle sizes of smaller than about 100 micron. The process comprises passing molten urea and molten sulphur through a mixing device at a temperature above the melting points to produce a finely divided sulphur dispersed in urea with the molten sulphur added in amounts sufficient to produce said urea sulphur fertilizer, maintaining a pressure drop across said mixing device of at least about 200 kPa to form a homogenized melt of urea sulphur, and solidifying said homogenized melt by prilling or agglomeration
Moreover, U.S. Pat. No. 4,394,150 teaches particles formed by passing a mixture of molten sulphur and a particulate swelling clay through a plurality of orifices and into an aqueous solution of a water soluble electrolyte salt, maintained at a temperature effective to cool the mixture below its solidification temperature.
Also, U.S. Pat. No. 4,569,859 relates to a process for prilling a mixture of sulphur and bentonite to produce a water-degradable prill by adding dry bentonite in powdered form to molten sulphur at a temperature to provide a molten sulphur-bentonite mixture, forming droplets of the mixture, providing a quenching solution of either sodium chloride, sodium sulphate, potassium chloride or potassium sulphate at a temperature low enough to solidify the sulphur-bentonite mixture.
Yet another particulate sulphur based fertilizer is taught in U.S. Pat. No. 5,571,303 comprising a uniform dispersion of sulphur and at least one member selected from the group consisting of ammonium sulfate, ammonium phosphate and mixtures thereof.
Furthermore, U.S. Pat. No. 5,599,373 relates to a sulphur-based, chemical soil-corrective or conditioner for agricultural use in the form of pellets, said product containing fly sulphur powder, at least 3% by weight of an inert compound selected from the group consisting of clay, bentonite, kaolin and mixtures thereof, and at least 0.5% by weight of a wetting agent.
Other fertilizers, compositions and method of manufacture can be found in U.S. Pat. Nos. 5,599,373, 5,571,303 and 4,330,319.
The present invention alleviates the safety problems associated with handling powdered sulphur and the problem of very slow conversion to sulphate of sulphur bentonite granular sulphur.